Plan for more time in this very fine park, although one day is better than nothing. You can easily spend a day just rock climbing, although I realize the possibility that rock climbing may not be your favorite activity..
Seriously, you can easily spend half a day in the visitor center. i would recommend a full day in the park after that when you have picked your favorite activity.
I always emphasize the VC because it is specifically designed to orient the visitor to the park's highlights and important resources. You will be greeted by a smiling, friendly, empathetic NPS ranger (almost always) who will have good answers to all your questions...
Cadillac mountain has really spectacular views and your can drive most of the way up. Also, there is an island a short way out from shore that you can only hike to at low tide, hike around it for a while, then head back. I don't remember the name of it unfortunately, but I'm sure they can point you to it at the visitor center as @hikermor suggested.